Buy as many as you like — set the quantity and add them to your cart. Once they're on your account, use them at the crate area in spawn.
Keys and ranks grouped together for less than buying them one at a time. The saving on each is worked out from the individual prices.
Worth being straight about, because it's your money.
Crate contents are randomised. The drops listed on each key are the pool it pulls from, not a guarantee of what you'll get on any single open.
A more expensive key doesn't guarantee a jackpot — it means the whole pool it draws from is stronger, so the floor is higher too.
Keys are for fun. Don't spend money you need elsewhere, and if you're under 18, ask whoever pays the card first.